Optimized uplink efficiencies for transmission of satellite data

ABSTRACT

Systems and methods for characterizing amplifiers. A system for characterizing an amplifier in accordance with the present invention comprises a Gaussian signal source for generating a signal in the frequency domain, a notch filter, coupled to the Gaussian Noise source, wherein the notch filter has a notch at a specified frequency and a frequency bandwidth, the frequency bandwidth encompassing the specified frequency, an Inverse Fast Fourier Transform device, coupled to an output of the notch filter, a normalization device, coupled to the Inverse Fast Fourier Transform device, an amplifier under test, coupled to the normalization device, for amplifying the signal generated by the Gaussian signal source, and a measurement device, coupled to an output of the amplifier, for measuring a power output of the amplifier in the frequency bandwidth and a noise output at the specified notch frequency, and for calculating the ratio between the power output and the noise output.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. utility patent applicationSer. No. 11/870,955, filed Oct. 11, 2007, which claims the benefit under35 U.S.C. Section 119(e) of U.S. provisional patent application Ser. No.60/852,140 filed Oct. 17, 2006, entitled “OPTIMIZED UPLINK EFFICIENCIESFOR TRANSMISSION OF SATELLITE DATA,” by Guangcai Zhou et al., whichapplication is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a satellite broadcast system,and in particular, to optimizing transmitter uplink power efficiency forsuch a satellite broadcast system.

2. Description of the Related Art

Satellite broadcasting of communications signals has become commonplace.Satellite distribution of commercial signals for use in televisionprogramming currently utilizes multiple feedhorns on a single OutdoorUnit (ODU) which supply signals to several Integrated Receiver &Decoders (IRDs) on separate cables from a multiswitch.

FIG. 1 illustrates an example satellite television broadcast system ofthe related art.

System 100 uses signals sent from Satellite A (SatA) 102, Satellite B(SatB) 104, and Satellite C (SatC) 106 (with transponders 28, 30, and 32converted to transponders 8, 10, and 12, respectively), that axedirectly broadcast to an Outdoor Unit (ODU) 108 that is typicallyattached to the outside of a house 110. ODU 108 receives these signalsand sends the received signals to IRD 112, which decodes the signals andseparates the signals into viewer channels, which are then passed totelevision 114 for viewing by a user. There can be more than onesatellite transmitting from each orbital location.

Satellite uplink signals 116 are transmitted by one or more uplinkfacilities 118 to the satellites 102-106 that are typically ingeosynchronous orbit. Satellites 102-106 amplify and rebroadcast theuplink signals 116, through transponders located on the satellite, asdownlink signals 120. Depending on the satellite 102-106 antennapattern, the downlink signals 120 are directed towards geographic areasfor reception by the ODU 108.

Each satellite 102-106 broadcasts downlink signals 120 in typicallythirty-two (32) different sets of frequencies, often referred to astransponders, which are licensed to various users for broadcasting ofprogramming, which can be audio, video, or data signals, or anycombination. These signals have typically been located in the Ku-bandFixed Satellite Service (FSS) and Broadcast Satellite Service (BSS)bands of frequencies in the 10-13 GHz range. Other satellites may alsobroadcast in a portion of the Ka-band with frequencies of 18-21 GHz.Alternatively, signals can be transmitted in series or in parallel viacable 122 to IRD 112.

The uplink signals 116 power is limited and frequency allocations arefixed in size and, thus, the power and bandwidth for transmission inthose frequency bands limit the ability of the system 100 to transmitadditional signals using the current transmission and receiving method.With additional satellites and additional channels, more signals must betransmitted within the power and bandwidth constraints to allow foradditional programming within system 100.

It can be seen, then, that there is a need in the art for a satellitebroadcast system to increase the power and spectral efficiency with theallocated bandwidth.

SUMMARY OF THE INVENTION

To minimize the limitations in the prior art, and to minimize otherlimitations that will become apparent upon reading and understanding thepresent specification, the present invention discloses systems andmethods for characterizing amplifiers in a multi-carrier environment. Asystem for characterizing an amplifier in accordance with the presentinvention comprises a Gaussian signal source for generating a signal inthe frequency domain, a notch filter, coupled to the Gaussian Noisesource, wherein the notch filter has a notch at a specified frequencyand a frequency bandwidth, the frequency bandwidth encompassing thespecified frequency, an Inverse Fast Fourier Transform device, coupledto an output of the notch filter, a normalization device, coupled to theInverse Fast Fourier Transform device, an amplifier under test, coupledto the normalization device, for amplifying the signal generated by theGaussian signal source, and a measurement device, coupled to an outputof the amplifier, for measuring a power output of the amplifier in thefrequency bandwidth and a noise output at the specified notch frequency,and for calculating the ratio between the power output and the noiseoutput.

Such a system further optionally comprises the characterized amplifierbeing used in a satellite communications system, and the satellitecommunications system is a direct broadcast system for televisionsignals.

A method in accordance with the present invention comprises supplying aGaussian signal in the frequency domain, notch filtering the Gaussiansignal, wherein the notch filter has a notch at a specified frequencyand a frequency bandwidth, the frequency bandwidth encompassing thespecified frequency, applying an Inverse Fast Fourier Transform to thenotch filtered Gaussian signal to transform the Gaussian signal into atime domain signal, normalizing the time domain signal to a specifiedpower level into the amplifier, applying the normalized time domainsignal to an input of the amplifier under test for its nonlinear effectson a multi-carrier signal, measuring a power output of the amplifier inthe frequency bandwidth and a noise output at the specified notchfrequency, and calculating the ratio between the power output and thenoise output.

Such a method further optionally comprises the characterized amplifierbeing used in a satellite communications system, and the satellitecommunications system is a direct broadcast system for televisionsignals.

Other features and advantages are inherent in the system and methodclaimed and disclosed or will become apparent to those skilled in theart from the following detailed description and its accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 illustrates a typical satellite television broadcast system ofthe related art;

FIG. 2 illustrates a typical measurement of C3IM;

FIG. 3 illustrates a typical measurement of ACPR;

FIG. 4 illustrates a measurement of NPR in accordance with the presentinvention; and

FIG. 5 illustrates a typical apparatus for measuring NPR in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings which form a part hereof, and which show, by way ofillustration, several embodiments of the present invention. It isunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the present invention.

Overview

The present invention provides a new figure-of-merit to measure theefficiency and suitability for a transmitter in a communications system.Traditionally, the figures-of-merit used to characterize a transmitter,e.g., a High Power Amplifier (HPA) or Travelling Wave Tube Amplifier(TWTA), is the third order intermodulation product, known as C3IM, andthe Adjacent Channel Power Ratio (ACPR).

However, other figures-of-merit may provide more efficiency for a givensystem than C3IM or ACPR. For example, Noise-Power Ratio (NPR), asdescribed herein in the present invention, may help to ensure enougheffective power from the TWTA or HPA to still transmit the data withoutexcessive errors to the satellite 120, which will help to ensure properand acceptable reception of the transmitted data at IRD 112.

Such an approach significantly increases the power and spectralefficiency of a system 100 that has a large number of transmitters.

Particulars of the Present Invention

FIG. 2 illustrates a typical measurement of C3IM.

Amplifier manufacturers typically supply data sheets for HPA and TWTAdevices that are specific to the device itself. For example, themanufacturer will run tests on the amplifier to give the end user somespecific data on how that amplifier will operate, the bandwidth, thepower output value, the non-linearity, etc. These data points willtypically be verified by the end user through testing at the userfacility. Non-linearity is typically expressed in terms of C3IM,

Graph 200 shows a series of relative output values 202A-202R, in dB, fora given amplifier. The horizontal axis of the graph is in MHz. Theinputs to this amplifier were two frequencies (two-tone), for examplef1=1.25 MHz and f2=2.5012 MHz. Output values were generated at variousfrequencies 204, with a center frequency at the center of thetransmission channel in system 200. Outputs 202J and 202K are at thetwo-tone input frequencies and are the highest power outputs for thisamplifier. 2f1-f2 and 2f2-f1 are the next highest outputs from nonlineareffects of the amplifier. Digitized amplifier output time samples areFast Fourier Transformed (FFT'd), and the input power is varied toobtain optimum tradeoff between output power and nonlinearity from thegraph. The input power reduction to obtain the desired output linearitywith a reduced output power is called the Input Back-Off (IBO), which isthe ratio between the input power that delivers maximum power and theoperating input power that delivers the desired nonlinearity tradeoff.IBO values typically range from 0 to 25 dB.

To determine C3IM for this amplifier, the difference between the highestpeak, in this case 202J and 202K, and the next adjacent highest peak, inthis case 202H and 202L, gives the C3IM value 208, measured in dB.

FIG. 3 illustrates a typical measurement of ACPR with a modulated inputsignal.

Graph 300 illustrates the output power 302 of a given amplifier, with aninput of a Quadrature Phased Shift Keyed (QPSK) signal with a symbolrate of 20 MHz and a Root Raised Cosine Factor (RRCF) of 0.2. Thesampling frequency for this example is 320 MHz, which is sufficient tocover most significant nonlinear effects in the signal spectrum. Thehorizontal axis of the graph is in MHz. The in-band Power SpectralDensity (PSD) 304 is shown, and the Upper Adjacent Channel PSD 306 andLower Adjacent Channel PSD 308 are also shown.

The ACPR is determined by taking the in-band PSD 304 and dividing by thesum of the Upper Adjacent Channel PSD 306 and Lower Adjacent Channel PSD308.

FIG. 4 illustrates a measurement of NPR in accordance with the presentinvention. The horizontal axis is in MHz and the vertical axis is in dB.

Graph 400 illustrates the output 402 of an amplifier with a zero-meanand unit-variance Gaussian input generated in the frequency domain,sampled at 160 MHz. The sampling rate can be varied as desired. A notchfilter is applied to the input signal, with a zero (null) at or close toDC, and a 1 elsewhere from −10 MHz to 10 MHz. An Inverse Fast FourierTransform (IFFT) transforms the digitized input to the time domain fortesting the nonlinearity of the amplifier

The NPR is calculated as the power at point 404 divided by the power atpoint 406.

FIG. 5 illustrates a typical apparatus for measuring NPR in accordancewith the present invention.

System 500 shows a Gaussian source 502, a notch filter 504, an IFFT 506,a normalization block 508, an amplifier/amplifier channel under test510, and the NPR calculation 512 with resultant NPR 514. The Gaussiansource 502 is used as an input, and can be adjusted in its power toeffect an IBO of 0 to 25 dB to allow for a NPR measurement at the outputof the amplifier 510. The Gaussian noise is 20 MHz wide, centered at 0Hz (and up-converted to the operating frequency of the amplifier), witha notch at or close to DC and a 1 from −10 MHz to +10 MHz. The notch canappear anywhere in the bandwidth, at any specified frequency orfrequency range, but is typically centered within the Gaussian noisebandwidth. The notch bandwidth can be specified as desired.

The Gaussian Source 502 output is fed through the notch filter 504 andthen transformed to the time domain by the IFFT 506. The output in thetime domain is normalized by normalization block 508, and then input tothe amplifier/amplifier channel 510. The graph of FIG. 4 is the result,where point 406 is generated by the notch at DC of filter 504, and thenormalized output 404 is generated by the amplifier 510.

The NPR calculation takes points 404 and 406, which take into accountthe nonlinearities of the amplifier 510 for a multi-carrier signal, anddetermines the output power of the amplifier 510 with respect to thepower of the input signal 502 as well as the inherent non-linearity ofthe amplifier 510 as it impacts a multi-carrier signal. Othermeasurements do not take these factors into account, which means thatthe amplifier 510 is not measured with respect to the end usage ofamplifier 510, e.g., to transmit a multi-carrier signal to a satellite.

Because of the NPR 514 accounting for the non-linearities of the system100 and amplifier 510, amplifiers 510 used in system 100 can be arrangedand set such that the signal strength of the signal amplified by theamplifier 510 is within a given range of all other signals beingreceived by the satellite. Since satellites receive signals from variousamplifiers, and, possibly, various uplink sites, the dynamic range ofthe signals being received by the satellite must be small such that thesatellite receive electronics do not apply too much gain to the receivedsignals. The Automatic Gain Control (AGC) attached to the receiveantenna on the satellites can only amplify or attenuate signals over acertain range. Since NPR measurement 514 gives a measurement of poweroutput (for the data) versus the noise (of the amplifier and inputsignals combined), the power output of the amplifier 510 can be finelytuned such that all amplifiers 510 being used in the system can deliversignal strengths to the satellite that are within a certain dynamicrange and still deliver all of the data required without undesirableerror rates or inter-symbol interferences.

CONCLUSION

In summary, the present invention comprises systems and methods forcharacterizing amplifiers. A system for characterizing an amplifier inaccordance with the present invention comprises a Gaussian signal sourcefor generating a signal in the frequency domain, a notch filter, coupledto the Gaussian Noise source, wherein the notch filter has a notch at aspecified frequency and a frequency bandwidth, the frequency bandwidthencompassing the specified frequency, an Inverse Fast Fourier Transformdevice, coupled to an output of the notch filter, a normalizationdevice, coupled to the Inverse Fast Fourier Transform device, anamplifier under test for its nonlinearity, coupled to the normalizationdevice, for amplifying the time-domain signal originally generated bythe Gaussian signal source, and a measurement device, coupled to anoutput of the amplifier, for measuring a power output of the amplifierin the frequency bandwidth and a noise output at the specifiedfrequency, and for calculating the ratio between the power output andthe noise output in the notch filter.

Such a system further optionally comprises the amplifier being used in asatellite communications system, and the satellite communications systemis a direct broadcast system for television signals.

A method in accordance with the present invention comprises supplying aGaussian signal, notch filtering the Gaussian signal, wherein the notchfilter has a notch at a specified frequency and a frequency bandwidth,the frequency bandwidth encompassing the specified frequency, applyingan Inverse Fast Fourier Transform to the notch filtered Gaussian signalto transform the Gaussian signal into a time domain signal, normalizingthe time domain signal, applying the normalized time domain signal to aninput of the amplifier, measuring a power output of the amplifier in thefrequency bandwidth and a noise output at the specified frequency, andcalculating the ratio between the power output and the noise output.

Such a method further optionally comprises the amplifier being used in asatellite communications system, and the satellite communications systemis a direct broadcast system for television signals.

It is intended that the scope of the invention be limited not by thisdetailed description, but rather by the claims appended hereto and theequivalents thereof. The above specification, examples and data providea complete description of the manufacture and use of the composition ofthe invention. Since many embodiments of the invention can be madewithout departing from the spirit and scope of the invention, theinvention resides in the claims hereinafter appended and the equivalentsthereof.

1. A system for characterizing an amplifier across a frequencybandwidth, comprising: a signal source for generating a signal in thefrequency domain; a notch filter, coupled to the signal source, thenotch filter having a notch at a notch frequency; an amplifier undertest, coupled to the normalization device, for amplifying the signalgenerated by the signal source; and a measurement device, coupled to anoutput of the amplifier, for measuring a power output of the amplifierin the frequency bandwidth and a noise output at the notch frequency,and for calculating a ratio between the power output and the noiseoutput.
 2. The system of claim 1, wherein the signal source is aGaussian source.
 3. The system of claim 2, wherein the Gaussian sourceis upconverted to an operating frequency of the amplifier.
 4. The systemof claim 1, wherein the notch frequency is centered in the frequencybandwidth.
 5. The system of claim 4, wherein the notch has a specifiedbandwidth.
 6. The system of claim 1, further comprising an Inverse FastFourier Transform device, coupled to an output of the notch filter, suchthat the measurement device calculates the ratio in a time domain. 7.The system of claim 1, further comprising a normalization device,coupled to the Inverse Fast Fourier Transform device, for normalizing anoutput of the Inverse Fast Fourier Transform device.
 8. The system ofclaim 1, wherein the amplifier is used in a satellite communicationssystem.
 9. The system of claim 8, wherein the satellite communicationssystem is a direct broadcast system for television signals.
 10. A methodfor characterizing an amplifier across a frequency bandwidth,comprising: notch filtering a Gaussian input signal to the amplifier,wherein the notch filtering comprises notching the Gaussian input signalat a specified frequency across the frequency bandwidth; measuring apower output of the amplifier across the frequency bandwidth and a noiseoutput at the specified frequency; and calculating the ratio between thepower output and the noise output.
 11. The method of claim 10, whereinthe specified frequency is centered in the frequency bandwidth.
 12. Themethod of claim 11, wherein the specified frequency has a specifiedbandwidth.
 13. The method of claim 10, further comprising transformingthe notch filtered Gaussian signal into a time domain signal.
 14. Themethod of claim 13, further comprising normalizing the time domainsignal.
 15. The method of claim 10, wherein the amplifier is used in asatellite communications system.
 16. The method of claim 15, wherein thesatellite communications system is a direct broadcast system fortelevision signals.
 17. A measurement device for characterizing anamplifier across a frequency bandwidth, comprising: a Gaussian signalsource for generating a signal in the frequency domain, the Gaussiansignal source being upconverted to an operating frequency of theamplifier; a notch filter, coupled to the Gaussian signal source, thenotch filter having a notch at a notch frequency; and a measurementdevice, coupled to an output of the amplifier, for measuring a poweroutput of the amplifier across the frequency bandwidth and a noiseoutput at the notch frequency, and for calculating a ratio between thepower output and the noise output.
 18. The measurement device of claim17, wherein the power output and the noise output are measured in a timedomain.
 19. The measurement device of claim 17, wherein the Gaussiansignal source has an adjustable power output.
 20. The measurement deviceof claim 17, wherein the amplifier is characterized for use in asatellite communications system.